Generation of upwelling circulation under downwelling-favorable wind within bottom-attached, buoyant coastal currents
Journal
Journal of Physical Oceanography
Journal Volume
47
Journal Issue
10
Pages
2499-2519
Date Issued
2017
Author(s)
Abstract
A two-dimensional modeling study by Moffat and Lentz recently reported that downwelling-favorable wind can induce cross-shore upwelling circulation within a bottom-attached, buoyant coastal current. Here, we extend the problem to three dimensions. The driving mechanism and the sensitivity for the upwelling circulation are studied, using a primitive equation ocean model and an analytical model. After the initial downwelling adjustment that steepens the isopycnals and compresses the coastal current, the cross-shore flow can switch to steady upwelling circulation. This reverse circulation coincides with a vertically well-mixed water column and persists until interrupted by the arrival of river plume bulge from upstream. During the upwelling phase, the ageostrophic cross-shore flow follows the Ekman balance. The sense of cross-shore circulation is governed by a dimensionless parameter, the shear ratio, which measures the relative size of geostrophic shear and velocity shear supported by the wind in the shallow-water limit. Upwelling circulation occurs when the shear ratio is greater than one. This condition represents that, near the surface, the windintensified pressure gradient exceeds the maximum possible Coriolis force associated with the wind-forced, alongshore flow. The resulting upwelling circulation acts to slump the isopycnals to restore the geostrophic balance. Therefore, within a coastal current, decreasing wind stress in fact strengthens the upwelling circulation, as a weaker wind produces a weaker shear and thus increases the imbalance. This inverse relation holds until the wind is too weak tomix the water column. Based on the analyticalmodel, a regime classification for the cross-shore circulation under downwelling-favorable wind is proposed. An observational example is given.
Type
journal article